If armors or joints of communication cables, particularly those of communication cables for underground buried wiring are damaged or deteriorated, infiltration of water into the cables brings to impairment of electric transmission characteristics of the cables. Moreover, if a pinhole is present in the insulators of the cables, insulation is reduced by infiltration of water, and at last a short circuit takes place to sometimes bring about an accident of wire breaking.
To prevent circuit troubles due to infiltration of water into cables, development of waterproof communication cables has been made and a great number of communication cables filled with jelly-like filling materials came to be employed.
As the jelly-like filling materials, blends of petroleum jelly such as petrolatum, polybutene, atactic polypropylene (PP), mineral oil, various waxes and the like are generally employed, and they are publicly known. Especially for thick cables, there is no filling materials satisfying all the requirements such as waterproof properties, electrical properties, dropping properties, melting point, consistency and workability, and filling materials obtained by adding blends to petrolatum are mainly employed. From the viewpoints of cost and supply stability, however, formulations mainly using polybutene are advantageous. The “polybutene” referred to herein means polybutene in a narrow sense, namely, a liquid polymer of a pentamer to a pentacontamer obtained from a mixture of three isomers of n-1-butene, n-2-butene and isobutylene.
For the polybutene-based waterproof cable jellies, a great number of formulations have been proposed, and it is also known to use polyethylene (PE) wax, paraffin wax and microcrystalline wax as additives. The reason why polybutene is used is that the volume shrinkage after pouring of a jelly into the cable is minimized to impart flexibility to the cable, and an oil is added for the purpose of reducing cost and adjusting fluidity. The PE wax is used to harden the jelly by cooling and thereby prevent oozing of oil, and 1 to 20% of this wax is usually added.
In the filling work of a cable with a cable jelly having been melted at a high temperature, the temperature at which the molten cable jelly is cooled and hardened in the cable is desired to be as low as possible. The reason is that it is required to fill the cable with the cable jelly in low viscosity more rapidly and in every nook and corner. On this account, the crystallizing temperature (Tc) of the PE wax to determine the hardening temperature of the cable jelly is desired to be as low as possible.
On the other hand, the PE wax is also required to have oil retention properties to prevent oozing of the oil by hardening the oil and polybutene in the cable jelly. When Tc of the wax is decreased for the purpose of decreasing the hardening temperature of the cable jelly, there arises a problem that the oil retention properties are also reduced at the same time. The reason is that decrease of crystallinity or crystallizability of the wax to decrease the Tc results in lowering of oil retention properties.
It is an object of the present invention to provide an additive for a communication cable filler, which can satisfy both the filling workability and the oil retention properties of a cable jelly, and a communication cable filler containing the additive.